1. Field of the Invention
The present invention relates to a polymer having nonlinear optical properties and a method for synthesizing the same, and more particularly, to a host-guest polymer system suitable for use in the manufacture of optical devices for high-speed, high-capacity data transmission systems, a side-chain nonlinear optical polymer derived from the host-guest polymer system, and a method for synthesizing the side-chain nonlinear optical polymer.
2. Description of the Related Art
Recently, the fast development of optical devices for high-speed, high-capacity data transmission systems has highly increased the need for new materials suitable therefore. As such, research on nonlinear optical materials has been actively performed in various aspects. To date, inorganic crystals, such as LiNbO3 or InGaAsP, have been used as materials for high-speed optical devices for optical communications. These inorganic crystals have stable, high optical non-linearity. However, every step of synthesizing the inorganic crystals is complicated and time consuming, thereby increasing the manufacturing cost of the inorganic crystals.
Meanwhile, organic nonlinear optical materials, which were first developed about 20 years ago, and in particular, organic polymers, are advantageous over inorganic materials in terms of their synthesis and processing procedures, and in that their physical properties, including the processing temperature, refractive index, optical coefficient, conversion of absorption wavelength, and the like, are controllable according to the various requirements. Thus, many approaches to obtain organic nonlinear optical polymers are being currently undertaken. Organic chromophores have in their molecular structure conjugate linkages for electron mobility and dipoles that are highly responsive to an external electric field due to the introduction of electron donating and electron accepting groups. Such an organic chromophore is incorporated in any form into polymers to form organic nonlinear optical polymers.
Organic nonlinear optical polymers are categorized into one of four types according to the correlation between their nonlinear optical organic chromophore and polymer (G. A. Lindsay, xe2x80x9cSecond-Order Nonlinear Optical Polymers: An Overview,xe2x80x9d ACS Symp. Ser. 60, G. A. Lindsay and K. D. Singereds., ACS, 1995, chap. 1).
A first type is a host-guest polymer system where an organic chromophore is dispersed in a polymer matrix and it can be obtained in a very simple way. As long as the organic dye can be dispersed in the polymer matrix, the mobility of organic chromophore molecules increases within the polymer, so that the polymer system provides maximized a poling effect. However, its optical non-linearity is greatly reduced in the manufacture of optical devices at a high temperature due to its free molecular movement. And, as the amount of organic dye increases, the glass transition temperature (Tg) of the polymer system decreases, and light scattering occurs due to an agglomeration of organic chromophore molecules, thereby resulting in optical loss.
A second type is a side-chain polymer system. This side-chain polymer system is the result of efforts made to overcome the drawbacks of the host-guest polymer system. An organic chromophore is chemically bound to a polymer in order to prevent the agglomeration of the organic dye molecules and to provide the resulting polymer system with an appropriate Tg, for example, of about 150-200xc2x0 C., for nonlinear optical stability at high temperatures. However, since poling efficiency is the greatest near Tg at which molecular movement of the polymer molecules is highly active, the optical non-linearity obtained at 150xc2x0 C. or less is likely to disappear during manufacture of devices at about 100xc2x0 C. Also, poling at a temperature of 200xc2x0 C. or greater likely decomposes the organic chromophore (M. H. Lee et al., xe2x80x9cPolymeric Electrooptic 2xc3x972 Switch consisting of Bifuraction Optical Active Waveguides and a Mach-Zehnder Interferometer,xe2x80x9d IEEE J. on Selected Topics in Quantum Electronics, 7, 812, 2001).
A third type is a main-chain polymer system obtained by incorporation of a nonlinear optical organic chromophore into a polymer main chain. As can be expected from this structure, the main-chain polymer system has lower molecular mobility than the side-chain polymer system and provides poor poling effect, but its optical non-linearity is thermally stable.
A fourth type is a cross-linked polymer system. This type of polymer system is provided for enhancing the thermal stability of the organic nonlinear optical polymer after poling. The cross-linked polymer system is applied to the host-guest polymer system and the side-chain polymer system having a low Tg. The cross-linked polymer system is obtained by poling a nonlinear optical polymer and cross-linking the polymer so as to improve a poling efficiency and the nonlinear optical stability of the organic chromophore at a high temperature. As a result of cross-linking the polymer, the mobility of the organic chromophore molecules decreases, and the great optical nonlinearity can be maintained even at a high temperature. In general, the polymer chain is thermal- or photo-crosslinked in the presence of a catalyst. However, after the cross-linking reaction, the unreacted cross-linkers or catalyst remain and thus limits use of the cross-linked polymer system in optical devices (U.S. Pat. Nos. 5,420,172 and 5,776,374).
In conclusion, to provide high optical nonlinearity and stably maintain the non-linearity at a high temperature, it seems to be most ideal to obtain the optical non-linearity by poling in a host-guest polymer system to maximize the poling effect and then converting into a side-chain polymer system to increase the thermal stability of the poling effect. In U.S. Pat. Nos. 5,484,821, 5,290,824, and 5,112,881, an organic chromophore having a cinnamoyl group is dispersed in polyvinylcinnamate, polyvinylstyrylacrylate, or polyvinylchalcone, poled in the host-guest polymer system, and photo-crosslinked. However, according to these disclosures, limited kinds of polymer matrixes, although their synthesis is easy, are used, and it is inconvenient to chemically incorporate the cynnamoyl group into the organic chromophore.
Accordingly, the present invention provides a host-guest polymer system having nonlinear optical properties suitable for use in the manufacture of optical devices, which are enhanced to be physically, chemically, and optically stable and maintain its nature in the manufacture of optical devices.
The present invention also provides a side-chain nonlinear optical polymer with enhanced, thermally stable optical non-linearity at high temperature, by maximizing poling effect and enhancing thermal stability after poling.
The present invention also provides a method for synthesizing the side-chain nonlinear optical polymer with enhanced, thermally stable optical non-linearity by maximizing poling effect and enhancing after poling.
In one aspect, the present invention provides a host-guest polymer system comprising: a polymer with an isoimide group having the following formula; and an organic chromophore having a hydroxy group or an amino group, 
In another aspect, the present invention provides a method for synthesizing a side-chain nonlinear optical polymer, the method comprising: forming a nonlinear optical polymer film based on a host-guest system in which an organic chromophore having a reactive group capable of nucleophilic reaction with an isoimide group is dispersed in a matrix including a polymer with the isoimide group having the following formula: 
poling the nonlinear optical film at a first temperature in an electric field; and reacting the organic chromophore with the polymer while poling at a second temperature which is higher than the first temperature, to synthesize the side-chain nonlinear optical polymer.
In the method for synthesizing the side-chain nonlinear optical polymer, the matrix includes polyisoimide having the isoimide group in its main chain. Alternatively, the matrix includes a polymer having the isoimide group in its side chain.
In the method for synthesizing the side-chain nonlinear optical polymer, the matrix is formed of a homopolymer having a repeating monomer unit, which has the isoimide group in its main or side chain, a copolymer having the repeating monomer unit, or a polymer blend containing the copolymer having the repeating monomer unit and the homopolymer.
In the method for synthesizing the side-chain nonlinear optical polymer, the organic chromophore comprises xcfx80-conjugated organic molecules. The organic chromophore comprises organic molecules having an electron donating group and an electron accepting group. The organic chromophore comprises a hydroxy group or an amino group which can be reacted with the isoimide group in the polymer matrix.
The first temperature is in the range of 100-200xc2x0 C.
In another aspect, the present invention provides a side-chain nonlinear optical polymer synthesized using the method described above.
The side-chain nonlinear optical polymer according to the present invention provides enhanced optical non-linearity suitable for manufacturing optical devices. The optical non-linearity of the side-chain polymer according to the present invention is thermally, physically, chemically, and optically stable, and thus its optical nature can be maintained in the manufacture of the optical device. The side-chain polymer according to the present invention is synthesized by maximizing poling effect and enhancing thermal stability, and thus its very high optical non-linearity can be maintained at high temperature.